[bbatsell]

Due to industrialization and cars, stormwater runoff, especially what’s called the first flush, is very polluted. If we had unlimited resources, treating both wastewater and stormwater runoff + strict impervious cover restrictions and on-site storm retention systems would be the best option. The reason a combined sewer isn’t generally preferred now is because the maximum capacity must account for torrential rain, otherwise the system will outflow raw sewage, and a polluted first flush is preferable to raw sewage. The cost to retrofit London’s sewer to be separated would be several orders of magnitude more than upsizing the combined sewer to handle predicted rain events. If they got the capacity right, then it is the best option.

[kypro]

Makes sense. Cheers for adding some more context.